A radio communication terminal includes a whip antenna utilized generally in a pulled-out position and an accommodated position. This whip antenna 130 has a helical antenna 112 at its tip. When the whip antenna is accommodated within a case 101 of a radio communication terminal 110 as shown in FIG. 24, only helical antenna 112 is exposed at case 101. In this case, a feeding point 115 is located at the base of helical antenna 112 as shown in FIG. 25. In other words, only helical antenna 112 serves as an antenna. The electrical length of the antenna in this case is substantially equal to the length of a straightened conductor forming the helical coil.
In contrast, when whip antenna 130 is pulled out as shown in FIG. 26, the base of an exposed whip portion 113, that is, the base of a linear portion serves as feeding point 115. An electrical length LE in this case is substantially equal to the length of the portion from feeding point 115 to the base of the helical antenna in the pulled-out direction as shown in FIG. 27. The antenna in this case is formed from pulled-out whip portion 113. Therefore, as long as the electrical length is adapted to the wavelength of the radio wave to be used, a conversation capacity should be obtained in both cases where the whip antenna is pulled out and the whip antenna is accommodated within the case. In this context, “adapting the conductor's length or the like to the wavelength of the radio wave” means that the electrical length of that conductor is set to have a length corresponding to a prescribed wavelength or a prescribed fraction of that wavelength.
A better communication performance, however, can be achieved when the whip antenna takes the pulled-out state than in the accommodated state because: (a) the likelihood increases of attaining a state away from an interfering environment such as the user's face or head in the pulled-out; and (b) a linear antenna often has higher radiation efficiency than a helical antenna state.
The whip antenna as described above also utilizes the ground in the mobile telephone to operate as a dipole antenna. In transmission or reception, an excitation current is induced at this dipole antenna by a radio wave (FIG. 28). An antenna at which a larger excitation current is induced by the same radio wave is regarded to have higher communication performance. As shown in FIG. 29, the excitation current is distributed over the whip antenna and the ground in the mobile telephone when the whip antenna is pulled out.
In general, in an area where the electric field is strong, such as in an urban area with a base station located nearby, a strong radio wave is transmitted and received between the base station and an antenna device. Therefore, even when the whip antenna is accommodated within the case, it can transmit and receive radio waves without any problems. In an area far away from the base station where the electric field is weak, however, the whip antenna may not be able to transmit and receive radio waves smoothly if it is accommodated within the case. That is, since the radio communication terminal is usually carried by a user with the whip antenna accommodated within the case, it may not be able to detect reception, for example. In addition, in view of the portability or the like, the idea of forcing customers to carry mobile telephones with the whip antennas always pulled out is not acceptable.
An approach set forth below has been taken to solve the above-described problems. As shown in FIG. 30, a mobile telephone has been proposed that has a structure in which a hand strap 101 and an antenna 102 are integrated and attached to a mobile telephone 110, or a structure in which the antenna itself serves as a hand strap (Japanese Utility Model Laying Open No. 6-7305). According to this configuration, the hand strap itself or a portion embedded in the hand strap substitutes for a pulled-out portion 104 of an antenna. Since the hand strap is always provided outside a case 103, the state in which the whip antenna is always pulled out can be realized in the mobile telephone as described above. In addition, a user would not feel uncomfortable about the mobile telephone with the hand strap attached at the external side of the case. Therefore, radio waves can be transmitted and received without any problems even in the area where the electric field is weak.
In the antenna's configuration as described above, the overall performance of the antenna will depend on the antenna incorporated into the hand strap. In other words, the whole antenna is located within the hand strap.
The performance of an antenna varies widely depending on the environment surrounding the antenna. In addition, the environment for a hand strap varies depending on how the user carries his/her mobile telephone. Therefore, the overall performance of the antenna may be affected by the way that the user carries his/her mobile telephone. Accordingly, a possibility of the overall communication performance being influenced by the user's way of carrying his/her mobile telephone cannot be denied.
Mobile telephones are utilized by all people regardless of sex and age, at any time day and night, and in any environment. Therefore, it is not preferred that an antenna affecting the communication performance depends on such a configuration as described above. There is a need for ensuring high communication performance even in a weak-electric-field area by means of the more stable configuration without any discomfort on the user's side.
Apart from the problems as discussed above, there is sometimes a need for improving only the communication performance of an existing radio communication device in a short preparation time while taking advantage of most of its characteristics. In other words, it is sometimes desired to make improvements in communication performance in a short preparation time by making only slight changes to the design of the conventional radio communication device. Conventionally, when improvements in communication performance are required, changes have been made to the specification of the circuit of a radio unit connected to an antenna or an antenna's system. Alternatively, instead of changing the antenna's system, at least its shape has been changed significantly. In such a way of improving the antenna's performance, circuitry of the radio communication device had to be designed again or a new mold must be fabricated again. Therefore, the preparation was often time-consuming and it sometimes took a long time before the products became available in the market.